1. Field of the Invention
The present invention relates to electro-optic displays and manufacturing methods thereof, and particularly to an active-matrix electro-optic display having thin film transistors (TFTs) as switching elements, and to a manufacturing method thereof.
2. Description of the Background Art
As electro-optic displays using liquid crystal or organic-electroluminescence (EL) as electro-optic elements, active-matrix TFT-array substrates (active-matrix substrates) are widely used in which an array of switching elements, such as thin film transistors, is formed on the substrate and an independent video signal is applied to each display pixel.
In such an electro-optic display, it is important to make the display area of each pixel as large as possible, or to use a substrate with a high aperture ratio, in order to obtain bright and high display quality.
Japanese Patent Application Laid-Open No. 10-170951 (see FIGS. 1 and 2, hereinafter referred to as Patent Document 1) discloses a common structure of such an active-matrix substrate with a high aperture ratio.
In the active-matrix substrate disclosed in the Patent Document 1, gate signal lines, gate insulating film, semiconductor film, and source and drain electrodes electrically connected to the semiconductor film are sequentially formed to fabricate TFTs on a transparent insulative substrate, e.g., glass. Then, the entire substrate including the TFTs is covered with an inorganic insulating film and further with an organic interlayer insulating film, which is followed by planarization. This structure allows the pattern of pixel electrodes to be overlapped with signal lines. This enhances the aperture ratio of the liquid crystal display and makes it possible to shield electric fields caused by signal lines.
In the active-matrix substrate disclosed in the Patent Document 1, a contact hole for electrically connecting the pixel electrode and the drain electrode of the TFT located under the pixel electrode is formed by etching the inorganic insulating film using the organic interlayer insulating film as a mask.
In this case, the edges of the inorganic insulating film that define the bottom of the contact hole are located in a position further inside in the plane direction than the edges of the organic interlayer insulating film existing thereon, and then the organic interlayer insulating film may project like eaves beyond the inorganic insulating film.
This phenomenon occurs because the inorganic insulating film is somewhat over-etched so that the drain electrode is exposed in the bottom of the contact hole. Then, the pixel electrode cannot cover the eave-like portions at the bottom of the contact hole, which results in disconnection.
Also, in a liquid crystal display, electrically connecting an upper electrode and a lower electrode through a contact hole passing through an interlayer insulating film may encounter another problem as described in Japanese Patent Application Laid-Open No. 2004-233683 (see FIG. 12(b): hereinafter referred to as Patent Document 2).
That is, according to the Patent Document 2, when the pixel electrode is made of transparent material such as ITO (Indium Tin Oxide) or IZO (Indium Zinc Oxide), the pixel electrode is prone to crack and disconnect on the organic interlayer insulating film at the peripheral edges of the contact hole.
In order to solve this problem, Patent Document 2 discloses a partially two-layered structure in which first and second pixel electrodes are stacked in the contact hole and its vicinity.
The inventors of the present invention examined the above-described problems of conventional liquid crystal displays and found that the formation of cracks on the organic interlayer insulating film at the peripheral edges of the contact hole can be avoided by gently tapering the peripheral edges of the contact hole or by forming at least the first pixel electrode not with oxide such as ITO and IZO but with ductile metal, for example.
Even when the first pixel electrode is made of a film of opaque metal, no problem arises with light transmission in the pixel area when the second pixel electrode is made of transparent material, because the first pixel electrode is disposed only in the contact hole and its vicinity.
However, it has been found that, when the first pixel electrode as the lower layer of the partially two-layered pixel electrode is made of a metal film, the edges of the metal film are likely to be reversely tapered in cross section, and then the overlying second pixel electrode may be disconnected in the reversely tapered portion.
The metal film is reversely tapered in cross section because the metal film is etched by etching liquid penetrating the interface between the metal film and the underlying organic interlayer insulating film.